Linear α-olefins are versatile intermediates in the preparation of a wide range of chemical products. For example, C4-C8 α-olefins are used as co-monomers with ethylene in copolymer formation, C12-C20 α-olefins are used as feedstock in surfactant formation and C6-C10 α-olefins are used as feedstocks for plasticizer formation.
Most commercially produced α-olefins are made by the oligomerization of ethylene in the presence of catalysts that tend to produce a mixture of α-olefins ranging from C4 to C30 and beyond. Examples of such commercially used catalysts include certain nickel-phosphine complexes, alkylaluminum compounds, and titanium halide with a Lewis acid.
In U.S. Pat. No. 6,103,946, for example, α-olefins are prepared from ethylene with an iron complex of a selected 2,6-pyridine dicarboxaldehyde bisimine and in some cases a selected activator such as an alkyl aluminum compound. The Schulz-Flory distribution (See B. Elvers, et al., Ed. Ullmann's Encyclopedia of Industrial Chemistry, Vol. A13, VCH Verlagsgesellschaft mbH Weinheim, 1989, p. 243-247 and 275-276) for the mixture of olefins prepared by that process is in the range of 0.7 to 0.87. The Schulz-Flory distribution is defined as:                α=n(Cn+2 olefin)/n(Cn olefin) wherein n(Cn olefin) is the moles of olefin containing n carbon atoms and n(Cn+2 olefin) is the moles of olefin containing n+2 carbon atoms. For α values in the range of 0.70 to 0.87 the α-olefins are in the C4 to C20 and greater range.        
Britovsek et al in Chem. Eur. J. 2000, 6, No. 12 pages 2221-2231 disclose the oligomerization of ethylene with a series of bis(imino)pyridyliron and cobalt complexes containing imino-aryl rings with methyl substituents in various positions on the aryl rings. In the majority of instances the α-values obtained were in the 0.63 to 0.79 range. In one instance in which the catalyst had only one methyl group in the ortho position the resulting mixture had an α-value of 0.50 with oligomer carbon numbers in the mixture ranging up to about 22.
Because the market demand for individual α-olefins making up these mixtures is not the same it would be useful to be able to vary the distribution to produce those olefins which are in greater demand which presently are those in the C4 to C10 range.
One object of the present invention is to produce linear a olefins from ethylene with an α-value of below about 0.45.
Another object is to produce substantial amounts of linear α-olefins from ethylene having carbon contents of from about C4 to about C10 and no α-olefins greater than C12.
These and other objects will become apparent from the description of the invention which follows.